Although it has been possible to produce optoelectronic devices, such as light emitting diodes (LEDs), that emit light in the deep ultra-violet (UV) wavelengths (λ≤280 nm) using group III metal nitride semiconductor materials, such as aluminium gallium nitride (AlGaN), the optical emission intensity from such LEDs to date has been relatively poor compared to visible wavelength LEDs. This is partly due to an inherent limitation in the AlGaN semiconductor material electronic band structure. It is found that the emission of deep ultraviolet light from crystalline AlGaN films in a direction substantially parallel to the layer formation growth axis is not favourable in traditional LED structures. In particular, deep ultraviolet LEDs are traditionally formed using a high aluminium content AlGaN alloy in order to obtain the required bandgap for the desired optical emission wavelength. Such high aluminium content compositions are particularly affected by the aforementioned limitation.
It has been widely believed that a poor deep ultraviolet emission intensity in such LEDs is due to an inferior crystalline structural quality of deposited group III metal nitride materials which leads to poor electrical behaviour of the LEDs. In comparison with other technologically mature group III-V compound semiconductors, such as gallium aluminium arsenide (GaAlAs), the group III metal nitrides exhibit crystalline defects at least two to three orders of magnitude higher. The structural quality of the group III metal nitrides can be improved by epitaxial deposition on native substrates, such as, aluminium nitride (AlN) and gallium nitride (GaN). However, even if AlN substrates are available, the deep ultraviolet LED formed using high aluminium content AlGaN materials is still unable to emit light efficiently in a vertical direction (i.e., parallel light emission perpendicular to the plane of the layer).
Yet a further problem exists in the prior art for operation of LEDs based on group III metal nitrides. The highest crystalline structure quality of group III metal nitride materials is formed using wurtzite crystal structure type films. These films are deposited on native or dissimilar hexagonal crystal symmetry substrates, with the so called c-plane orientation. Such c-plane oriented group III metal nitride films have the unique property of forming extremely large internal charge sheets at the interface boundary of two dissimilar AlGaN compositions. These charges are called pyroelectric charges and appear at every layer composition discontinuity. Furthermore, each and every different AlGaN composition possesses a slightly different crystal lattice parameter, and therefore each dissimilar AlGaN layer readily forms crystal misfit dislocations at the interface boundary which propagate into the interior of the layer if not correctly managed. If the dissimilar AlGaN layers are formed to minimize the crystal misfit dislocations, then yet another problematic internal charge is generated, called a piezoelectric charge. These internal pyroelectric and piezoelectric charges therefore impose further challenges to LED design as they generate internal electric fields within the LED that tend to oppose the recombination of the charge carriers that is required for light generation.
A further problem is the inherently high refractive index of group III metal nitride materials which further limits the amount of the light generated within the LED which can escape from the surface. Significant efforts have been made in surface texturing to improve an escape cone of light from the surface. These solutions have had some success by improving the light emission from deep UV LEDs but are still far from achieving optical power densities of commercial significance when compared to UV gas-lamps technologies. Even with surface texturing, and the use of optical coupling structures, such as photonic bandgap patterned structures, UV LEDs have been unable to emit light efficiently in a vertical direction.
A yet further limitation found in the prior art is that in comparison to group III metal arsenide semiconductors, group III metal nitride semiconductors are extremely challenging to grow via film deposition. Even though a convincing range of arbitrary alloy compositions of indium gallium nitride (InxGa1-xN), aluminium gallium nitride (AlxGa1-xN) and indium gallium aluminium nitride (InxGayAl1-x-yN) have been demonstrated using both molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD), there remain large technical challenges in deposition a large number of dissimilar compositions as part of a single epitaxial stack of an LED. In practice, this limits the complexity and the range of bandgap engineered structures that can be realized using group III metal nitride semiconductors and such growth techniques.
There is therefore a need for an improved solid state optoelectronic device for use at UV frequencies, particularly deep UV frequencies. There is yet a further need to improve the film formation method for engineering such optoelectronic devices.